This documentation is for version v1.3.0 of NNG, but the latest released version is v1.8.0. see the documentation for v1.8.0 for the most up-to-date information.


#include <nng/nng.h>

int nng_recv(nng_socket s, void *data, size_t *sizep, int flags);


The nng_recv() receives a message.

The flags is a bit mask that may contain any of the following values:


The function returns immediately, even if no message is available. Without this flag, the function will wait until a message is received by the socket s, or any configured timer expires.


If this flag is present, then a “zero-copy” mode is used. In this case the caller must set the value of data to the location of another pointer (of type void *), and the sizep pointer must be set to a location to receive the size of the message body. The function will then allocate a message buffer (as if by nng_alloc()), fill it with the message body, and store it at the address referenced by data, and update the size referenced by sizep. The caller is responsible for disposing of the received buffer either by the nng_free() function or passing the message (also with the NNG_FLAG_ALLOC flag) in a call to nng_send().

If the special flag NNG_FLAG_ALLOC (see above) is not specified, then the caller must set data to a buffer to receive the message body content, and must store the size of that buffer at the location pointed to by sizep. When the function returns, if it is successful, the size at sizep will be updated with the actual message body length copied into data.

The semantics of what receiving a message means vary from protocol to protocol, so examination of the protocol documentation is encouraged. (For example, with a req socket a message may only be received after a request has been sent, and a sub socket may only receive messages corresponding to topics to which it has subscribed.) Furthermore, some protocols may not support receiving data at all, such as pub.
The NNG_FLAG_ALLOC flag can be used to reduce data copies, thereby increasing performance, particularly if the buffer is reused to send a response using the same flag.


This function returns 0 on success, and non-zero otherwise.



The operation would block, but NNG_FLAG_NONBLOCK was specified.


The socket s is not open.


An invalid set of flags was specified.


The received message did not fit in the size provided.


Insufficient memory is available.


The protocol for socket s does not support receiving.


The socket s cannot receive data in this state.


The operation timed out.